1. Industrial Field of Utilization
This invention relates to an emulsion-type-rust-preventive baking composition which has good workability, can form a durable and surpassingly rust-preventive coat film on an article treated therewith and is particularly useful in the prevention of rusting of reinforcing steel rods for reinforced concrete.
2. Prior Art
Generally, steel rods for concrete reinforcement are used in two ways: they are assembled on the spot to give a concrete structure-reinforcing construction or used in factories as reinforcements for precast boards or autoclaved lightweight concrete products. In either moder of use, steel rods for concrete reinforcement encounter rusting-related problems. Thus, in the case of on-the-spot use, steel rods are exposed to the weather (wind, rain, sunlight) and get rusty until their embedment into concrete. When concrete placing is conducted with rusty steel rods, the steel rods fail to satisfactorily perform their proper reinforcing function. Recently, sea sand is used in large amounts as the sand for concrete, and salts contained in sea sand offer a problem; they cause corrosion of reinforcing steel rods, hence marked decrease in durability of concrete structures, even after concrete placing. Furthermore, concrete constructions built near the seashore are subject to corrosion of reinforcing steel rods as a result of permeation of salts. Such corrosion results in decrease in the strength of said concrete constructions and, in extreme cases, in disintegration of said constructions, in a short period of time.
On the other hand, in the case of use in factories, particularly in the case of use in autoclaved lightweight concrete products, reinforcing steel rods are yet very susceptible to rusting because the body concrete is not alkaline but kept neutralized and is porous and readily permeable to moist and air. Thus, rusting is a very serious problem to steel rods for concrete reinforcement. Development of effective means of rust prevention is earnestly desired.
In recent years, various techniques of preventing steel rods for concrete reinforcement from rusting have been proposed. They are classifiable roughly into four categories, namely (1) the technique comprising applying to reinforcing steel rods a cement-type rust-preventive composition which is a mixture of a synthetic resin or the like and cement or the like, (2) the technique comprising applying to reinforcing steel rods an organic solvent-based paint to thereby form a close and compact coat film on the surface of said rods, (3) the technique comprising forming a rust-preventive coat film on the steel rod surface using a water-based rust-preventive paint composed of a rubber latex, a silica powder, lime, etc. and (4) the technique comprising baking an epoxy resin powder on the steel rod surface. According to a typical example of the above technique (1), as described in Japanese Patent Publications Nos. 2843/1956, 23879/1961 and 15062/1970, methylcellulose or a latex is added to portland cement and the mixture is applied to reinforcing steel rods or, according to another example of technique (1), as described in Japanese Patent Publication No. 1698/1983, a cement-based rust-preventive composition is prepared from a synthetic resin emulsion and a rapid-curing cement and applied to such rods. However, these proposed category (1) techniques are not yet fully satisfactory in respect of reliability. The above-mentioned cement-based rust-preventive composition has a pot life and therefore is usable only within a limited period of time. The rust-preventive coat film is fragile and easily peels off during transportation of coated reinforcing steel rods, shrinkage of the coat film leads to crack or fissure formation, pinholes are producd on the coat film and, due to these phenomena, the rust-inhibiting power is apt to decrease, hence lacks reliability. Furthermore, when cement is neutralized, the rust-inhibiting power lowers to an extreme extent. According to a typical example of the above-mentioned category (2) technique, as described in Japanese Patent Publication No. 1581/1975, a close and compact coat film is formed by applying a polystyrene resin solution type paint to the reinforcing steel rod surface. This technique indeed produces excellent rust-preventive effect but can hardly be put into practical use because the solvent used there may possibly cause working environment impairment, fire, air pollution and/or environmental disruption, among others. According to a typical example of the above-mentioned category (3) technique, as described in Japanese Patent Publication No. 97617/1975, a water-based rust-preventive paint is prepared by admixing a silica powder, lime and so forth with a rubber latex and this paint is used for rust-preventive coat film formation. This technique is disadvantageous, however, since the rust-preventive coat film is inferior in closeness and compactness to that derived from the above-mentioned organic solvent-based paint, hence the rust-inhibiting power cannot reach a practical level. According to a typical example of the above-mentioned category (4) technique, an epoxy resin powder is applied to reinforcing steel rods by baking. This technique gives a very firm and strong rust-preventive coat film and therefore is the most reliable one among the known rust-preventive treatment techniques. However, this technique is disadvantageous because of expensiveness and insufficient adhesion of the coat film to body concrete.